If you think the only threat from the ocean due to global warming is sea-level rise, think again.

The ocean is absorbing over 90 percent of the heat trapped by greenhouse gases, and about half of the carbon dioxide (CO2) resulting from our burning fossil fuels. In water, CO2 forms carbonic acid, acidifying the ocean.

As the surface of the ocean warms, the water expands and becomes lighter, forming a layer that sits on top of the deeper, colder and more nutrient-rich water below. If the warmer surface layer is thin, mixing with the deeper water is able to occur as winds and storms churn the ocean surface. But as the warming extends, deeper mixing stops and stratification occurs.

Because the warmer surface water is unable to hold as much oxygen and the colder, oxygen- and nutrient-rich water is unable to rise, “dead zones” develop, particularly in shallow coastal areas. Most aerobic (oxygen breathing) marine life cannot survive in these dead zones.

Out of this warming, acidifying, nutrient-poor surface water come two serious threats: the loss of oxygen-producing phytoplankton and reef-building coral that shelter millions of species of marine life.

For half of Earth’s 4.6-billion-year history, the atmosphere contained almost no oxygen. Then, ocean-dwelling blue-green algae, a form of phytoplankton, began producing oxygen by photosynthesis, perhaps 3.5 billion years ago.

But most early oxygen did not reach the atmosphere; it was taken up by iron and other elements in the ocean, forming iron oxide and other oxidized elements. About 2.5 billion years ago there was little iron remaining to react with oxygen, and oxygen started entering the atmosphere. It took the next 2 billion years to reach approximately present oxygen levels.

Today, 21 percent of the Earth’s atmosphere is oxygen by volume. Less than 15 percent would be life-threatening. Different data sources vary, but somewhere between 50 to 70 or even 80 percent of the oxygen we breathe is attributed to phytoplankton.

Plankton are micro-organisms floating near the ocean’s surface to the depth sunlight penetrates. Plankton make up 98 percent of all ocean life, and it is said there are more plankton in the ocean than there are stars in the universe. Plankton includes two forms of life: plants called phytoplankton and animals called zooplankton.

Phytoplankton live off minerals in the ocean and photosynthesis of carbon dioxide (CO2) powered by the energy of the sun. In the chemical process of photosynthesis, oxygen is released to the atmosphere as a waste product.

Zooplankton feed off of the phytoplankton, and together they form the base of the marine food chain, upon which all ocean life depends, directly or indirectly.

A study by researchers at Canada’s Dalhousie University, published in the journal Nature (July 29, 2010), found the global population of phytoplankton had fallen about 40 percent since 1950, with scientists attributing the decline to rising sea surface temperatures.

A recent study by the National Center for Atmospheric Research (NCAR), published in the journal Global Biogeochemical Cycles, is summarized in an article in Atmos (April 27, 2016). The article states, “A reduction in the amount of oxygen dissolved in the oceans due to climate change is already discernible in some parts of the world and should be evident across large regions of the oceans between 2030 and 2040.”

Also, a recent article in Science News (August 6, 2016) discussed an experiment that simulated ocean acidification to test the ability of some phytoplankton (E. huxleyi) to build their protective shells. The experiment indicated a decline in phytoplankton shell building.

Given time, these organisms may be able to adapt, but warming, acidification and loss of nutrients may be occurring too fast to hold much immediate hope. It took 3 million years after the last mass extinction event for the ocean to recover to its pre-extinction state.

According to the National Oceanic and Atmospheric Administration, “corals are extremely ancient animals that evolved into modern reef-building forms over the last 25 million years.” Most corals have a symbiotic relationship with algae (a form of phytoplankton) that live within them: “…When corals are stressed by changes in conditions such as temperature, light or nutrients, they expel the symbiotic algae … causing them to turn completely white.” If the “bleaching” event lasts too long or occurs too frequently, the coral die.

According to the World Wildlife Fund “…Some estimates put the total diversity of life found in, on and around all coral reefs at up to 2 million species.

“The world is in the midst of a global bleaching event…” (The Guardian, April 19, 2016). This year, with unprecedented El Nino warming, Pacific coral has experienced significant bleaching and die-off. Australia’s Great Barrier Reef has suffered loss of up to 90 percent dead or dying in the northern third of its extent.

This is the third mass bleaching event to have occurred since 1998 — and the worst.

William Gran, now retired, was an adjunct instructor at Greenfield Community College on global warming and climate change. He can be reached at whgran@gmail.com